1. Field of the Invention
The present invention relates to an active matrix substrate incorporated in a liquid crystal panel and a method for producing the same.
2. Description of the Related Art
FIG. 16 is a circuit diagram showing a structure of a conventional active matrix liquid crystal display device.
Referring to FIG. 16, the conventional active matrix liquid crystal display device includes a plurality of gate lines 1 arranged in parallel to each other, a plurality of source lines 2 arranged in parallel to each other, a plurality of pixels 3 provided in a matrix and a common electrode 4. The gate lines 1 serving as scanning lines extend along rows of the pixels 3 so as to respectively correspond to the rows, while the source lines 2 serving as data lines extend along columns of the pixels 3 so as to respectively correspond to the columns. Therefore, the gate lines 1 and the source lines 2 cross each other to enclose a plurality of regions arranged in a matrix. Each of the pixels 3 is provided in each of the enclosed regions and is mainly composed of a pixel electrode 5 and a switching element 6, i.e., a thin film transistor (TFT). The switching element 6 is connected to a corresponding gate line 1 and a corresponding source line 2. Furthermore, the active matrix liquid crystal display device includes lead terminals 7 and 8 which connect the gate lines 1 and the source lines 2 to respective drivers.
Although not illustrated, a liquid crystal panel of such a liquid crystal display device usually includes a pair of substrates opposed to each other with a gap therebetween, the gap being filled with a liquid crystal material. One of the pair of substrates is referred to as an active matrix substrate which is provided with the above-mentioned gate lines 1, source lines 2 and pixels 3. The other substrate is referred to as a counter substrate which is provided with the above-mentioned common electrode 4. Furthermore, the counter substrate may be optionally provided with an RGB or a YMC color filter.
FIG. 17 is a plan view showing a one-pixel portion of an exemplary active matrix substrate 10. FIG. 18 is a cross-sectional view taken along line XVIII--XVIII of FIG. 17. FIG. 19 is a cross-sectional view taken along line XIX--XIX of FIG. 17.
The active matrix substrate 10 includes the pixel electrodes 5 which are respectively provided in pixel regions enclosed by the gate lines 1 and the source lines 2. In each pixel region, the switching element (TFT) 6 having a reversed staggered structure is also provided. As shown in FIG. 18, both of the pixel electrode 5 and the switching element 6 are formed on a transparent insulating substrate 11. Referring to FIG. 18, the switching element 6 includes a gate electrode 12, a gate insulating film 13, a semiconductor layer 14, a channel protection layer 15, a source electrode 16, a drain electrode 17, a first contact layer 18 and a second contact layer 19. The gate electrode 12, the source electrode 6 and the drain electrode 17 are connected to the gate line 1, the source line 2 and the pixel electrode 5, respectively.
As shown in FIGS. 18 and 19, the gate electrode 12 and the gate line 1 are provided beneath the gate insulating film 13 while the pixel electrode 5 and the source line 2 are provided above the gate insulating film 13. The active matrix substrate 10 having such a structure suffers from the following problems.
First, the source line 2 and the pixel electrode 5 provided above the gate insulating film 13 may be short-circuited with each other. Thus, short-circuit is caused by a conductive material piece existing between the source line 2 and the pixel electrode 5 so as to establish an electrical connection therebetween, resulting in a display defect. The conductive material piece is formed, for example, due to a patterning imperfection during a step of forming the source lines 2 and the pixel electrodes 5. In another case, the conductive material piece may be an etching residue or a reaction product which is left between the source line 2 and the pixel electrode 5 after formation the semiconductor layer 14 and the contact layers 18 and 19 is completed. The above-mentioned structural defect, i.e., the short-circuit, can be repaired by examining the active matrix substrate 10 after completing the whole production process so as to locate the structural defect, and laser radiating to the located structural defect. However, this radiation may spatter conductive material from the located structural defect to another place and cause another short-circuit. In addition, such a radiation treatment is complicated and increases production cost.
Second, when the etching residue or the reaction product which resulted from the formation of the semiconductor layer 14 or the contact layers 18 and 19 is present on the gate insulating film 13 above the gate line 1, an undesired capacitance may be formed by the gate line 1, the gate insulating film 13 and the etching residue or reaction product, and electrically connected to the pixel electrode 5. Such a structural defect, i.e., addition of the undesired capacitance, cannot be repaired after the production of the active matrix substrate 10. Thus, in the worst case, the produced active matrix substrate is treated as a waste product.
Finally, short-circuit may occur between the lead terminals 7 of the gate lines 1, or between the lead terminals 8 of the source lines 2, due to patterning imperfection.